The present invention relates to a communication channel selection method, and a wireless station and a wireless terminal employing it, and more particularly to an interference avoidance technique in a wireless network system.
A wireless LAN system to be represented by the 802.11 standard standardized in the Institute of Electrical and Electronics Engineers (hereinafter, described as the IEEE), which is generally configured of an access point (AP) for connecting a wireless network and a wire network, and a plurality of wireless terminals, in which mutual communication is made by employing one of a plurality of frequency channels that are available. In such a wireless LAN system, however, a shared frequency band requiring no license is generally utilized, whereby the possibility exists that neighboring other wireless LAN systems utilize an identical frequency channel. At this moment, interference between the neighboring wireless LAN systems that causes communication performance of its own system to degrade due to a radio wave to be transmitted by the other system. As one of means for avoiding this interference, a method is generally known of removing an interference status by changing the frequency channel that the wireless LAN system uses.
The foregoing interference avoidance method by the channel change was not specially specified in the conventional wireless LAN system (IEEE standard 802.11); however the AP packages means for detecting the interference, and means for changing the channel to be used, thereby making it possible to realize interference avoidance by changing the channel. In this case, the AP, which judged that the channel to be used had to be changed because interference was detected, can change its own channel; however it has no means for notifying it to the other terminal. Accordingly, when the AP made a channel change, the terminal is not able to recognize it, which causes a connection between the AP and the terminal to be disconnected. The terminal of which a connection was disconnected searches for the AP by scanning each channel for re-connection.
That is, the conventional wireless LAN system based upon the IEEE 802.11 standard had the problem that a communication disconnection time during which communication between the wireless stations became impossible to make because of the use channel change by the wireless LAN system became long.
So as to solve this problem, the IEEE standard 802.11h Potential Draft Text (02/245r2) to be cited as a non-patent document 1 defined a channel change notification message for notifying each terminal that the AP changed the channel. The time until the channel change and the change destination channel were shown in this channel change notification message. The terminal that received this channel change notification message can recognize when the AP changes which channel, which allows reduction of the communication disconnection time to be expected.
[Non-Patent Document 1]
EEE Standard 802.11h Potential Draft Text (02/245r2)
As described above, a format of the channel change notification message was specified in the non-patent document 1; however its transmission method was not specified. A transmission error exists intrinsically in the wireless communication, so the possibility exists that the terminal is not able to receive the channel change notification message from the AP correctly. The terminal that failed to receive the channel change notification message is not able to recognize the channel change by the AP, whereby it reconnects with the AP again after a lapse of a long communication disconnection time according to a process similar to that of the conventional wireless LAN system specified in the IEEE 802.11. That is, there is the problem that the terminal, which is not able to make a channel change at a timing that the AP intended due to the transmission error, can exist.